This invention relates to web processing equipment, and more particularly to machinery for preparing continuous business forms.
The preparation of continuous business forms has long provided unique and difficult challenges. The forms are prepared in substantial lengths and usually include several layers (e. g., a top original layer, an intermediate carbon, and a bottom copy). Each layer is normally separably joined to the others, and each individual form is joined to the next form in its own layer by lines of perforation, so that the forms may be used continuously and then separated one by one, as needed. This requires extreme accuracy in the manufacturing of these forms, since even the smallest dimensional errors may prove intolerably cumulative when repeated over the length of several hundred forms.
With conventional, single sheet (or single copy) printing, each page (or each few pages, as in a signature) is separated from the others, and at some point individually trimmed to size. In a typical press run, therefore, slight differences in size are customary and are accommodated routinely when the individual pages are sized.
Such is not the case, however, with continuous business forms. If the upper (original) copy of a business form is perhaps 0.001 inch longer than its corresponding duplicate (the one to be located beneath it), then a continuous roll of one thousand such forms will find the last upper (original) form shifted one inch out of registry with its corresponding lower (duplicate) form. Tolerances must actually be much closer than this, and it has therefore been common practice in the industry not to use conventional printing press methods, since they do not produce the accuracies required.
Much of the difficulty is in the paper itself: it is a physical material and therefore has a modulus of elasticity. It elongates when tensioned and returns when released. The amount is small but can be significant over the length of a roll of continuous forms. Similarly, variations in moisture content from one portion of the paper to another will lengthen the more moist portions. Typically, the edges of a roll of paper absorb moisture during storage, and the edges then run slack in the printing press. Also, the pressure upon the paper within the nip of the printing press elongates the paper transversely (crosswise).
In commercial presses for printing single pages, tensions from 2 to 10 pounds per inch are commonly used to take up the slackness in the web edges and to provide the desired web control. Shortening of the web when such tensions are released is not a problem since the pages are individually cut to size. Tensions are also used in the production of continuous business forms, but are usually limited to 1/2 to 3 pounds per inch in order to limit elastic contractions when the web is released.
To see that the web behaves the same from one stage to the next in the printing of continuous business forms, additional precautions are taken. These include carefully varying the tension from stage to stage in the printing press, storing the web rolls under precise temperature and humidity conditions, running an entire printing operation in a single day to reduce environmental fluctuations, and so on. Where appropriate, the various layers of the multiple layer business form are taken from the same portion of the paper supply roll. (That is, the roll is often double or triple width, so that all forms printed on paper drawn from the right side of the roll are collated with forms drawn from the right side, those from the left are collated with others drawn from the left, and so on.) As a result of these precautions, the web can normally be controlled with a high degree of accuracy. U.S. Pat. Nos. 3,249,316 and 3,592,133, assigned to the assignee of the present invention, describe machinery used for precision web control during the printing process.
If a given length of paper is metered into a press, it follows that a corresponding length of paper will issue from the press. The precision unwind drums disclosed in the above patents meter the paper into the printing press at precisely controlled rates. In some operations, an unwind drum may accordingly be selected to deliver fractionally more paper than desired, and an adjustable snubber is then used to shorten the paper by tensioning it as it is fed onto the drum. By tensioning the paper, the snubber stretches it slightly while it is being metered by the drum, so the effective length of the paper is reduced when the tension is later released. By regulating the snubber, extremely minute changes in the rate that the paper is fed into the press (and hence the length of the paper) can be made.
Occasionally, operating conditions shift sufficiently that normal adjustments of the machinery fail to supply the web at the proper rate. As an example, seasonal fluctuations in temperature and humidity can combine so that no acceptable amount of snubbing will adjust the web properly.
To understand this problem, it must be recognized that changes in the web thickness can affect the rate the web is supplied by the unwind drum. That is, the velocity of the paper web is a function of the "effective radius" of the unwind drum (for a given rate of rotation). The effective radius of the drum is the radius of the drum as measured with respect to the neutral axis of the paper web. This axis includes generally about one-half the thickness of the paper since it is the axis within the web (in the direction of its thickness) on which the web neither shortens (compresses) or elongates (stretches) as it is wound onto the unwind drum. Obviously, in assuming a curved configuration around the drum, the inner surface of the web will necessarily be shorter than the outer surface, so that some shortening or elongation of one surface or the other must take place. At some point within the web (between or on one of the surfaces) neither shortening nor elongation will take place, and this is the neutral axis of the web. Since the neutral axis of the web usually changes with changes in the web thickness, the rate the paper is supplied by the unwind drum can be altered by changes in the web thickness as well as by changes in the drum diameter. The greater the effective radius, the faster the web is supplied.
In an attempt to compensate for environmental fluctuations, electric heaters were used in one unwind drum to increase its effective diameter by means of thermal expansion. Another attempt involved splitting the drum in half and using a jack screw to vary the drum diameter. Still another attempt employed three drums, each of one-half size, and each of slightly different diameter, so that the appropriate drum for the conditions present could be used. Another early attempt involved use of an infinitely variable drive to drive a large size drum. None of these worked. The drum could not be heated satisfactorily; it could not be split accurately; triple drums proved expensive and impractical; and the infinitely variable drives were too coarse and tended to hunt when used to drive large size unwind drums.
Thus there is still a need for a way to vary the effective diameter of the unwind drum, as needed, by an extremely accurate and precisely controlled, yet very minute amount, and to reduce the sensitivity of the unwind drum to changes in the thickness of the paper webs fed thereon.